yep run a pressure check for sure, brewski is correct, these transmissions did do that when the clutch pack (usually forward) slipped.

One other thing to check is pull the floor plate and look at the control valve on top of the transmission, you'll see a cable hooked to a bracket, this cable hooks to a lever that is hooked to a spool valve (this is the inching spool), you want to check this lever assy and see if it is sticking or hard to move. Cat trucks had this problem and it is the orings on the shaft the lever mounts on, they get dry and cause the lever to drag not allowing the inching spool to move freely. Mitsubitshi trucks may not use orings but may have bushings. In their MCFA document #000021-09M they have a new service part for this which is an updated lever with a grease nipple on it and they recommend replacing with this and also replacing the bushings.

Unhook the spool and cable from this lever and remove the retaining ring or bolt holding the lever and remove it. You can try greasing it and reassembling and see if that helps any, but if it has much play in it you'll want to replace it. Inspect all the parts and if any are worn replace them, here is a list of inching lever assy parts:

Inching lever: part number 91A2860900
bushing: part number 91A2804300
retainer bushing: part number 91A2813300
inching lever bolt: part number 91A2813800

when you check the pressures you will find 3 small plugs on top of the control valve, they will use a small allen wrench, i suggest using a brass punch and pinging them first to pop them loose, they can be tight and may not want to come out, the allen wrench will strip the allen socket in the plug out if not careful.
- The plug closest to the torque converter just off center on the control valve is torque converter pressure, it should read 0.16 +/- 0.02 (1.63+/- 0.20 kgf/cm) (23.18 psi) at idle in neutral 600 rpm. Or 0.40 +/- 0.02 (4.08+/- 0.20 kgf/cm) (58.031 psi) at 1600 rpm.

-the plug just in front of the 1st one centered on control valve is main oil pressure,
600 rpm, neutral (1.12+/- 0.03 (11.42+/- 0.31 kgf/cm) (162.43 psi).
at high rpm 1600 it should read 1.26+/- 0.03 (12.85+/- 0.31 kgf/cm) (182.77 psi).

-the plug directly in the center of the control valve in front of the 2nd one is clutch oil pressure (this is the one you want to check first). It's pressure readings should be:
in forward at 600 rpm 1.09 +/- 0.03 (11.11+/- 0.31 kgf/cm) (158.02 psi)
in forward at 1600 rpm 1.21 +/- 0.03 (12.34 +/- 0/31 kgf/cm) (175.51 psi)
in reverse at 600 rpm 1.06 +/- 0.03 (10.81 +/- 0.31 kgf/cm) (153.75 psi)
in reverse at 1600 rpm 1.18 +/- 0.03 (12.03 +/- 0.31 kgf/cm) (171.10 psi)

You will most likely need a metric adapter fitting to hook a gauge to it. the size for the control valve port is M14 x 1.5 thread and the other side of the adapter will have to match your gauges thread, most are 1/8 rpt.

Good luck

Run pressure check. Usually when you have slipping in one direction it is due to worn clutch pack.